Although there have been improvements in efficiency of photovoltaic (PV) cells, their success as efficient energy producers is also dependent on how their outputs are interfaced with powered devices. PV cells are typically connected in series arrangements to obtain a desired voltage value, and then the series arrangements are connected in parallel to obtain a desired current level. These arrangements of PV cells and their electrical interconnections are placed on a support structure to form a PV panel. The energy available from the PV panel is dependent on the characteristics of the PV cells and the amount of light energy, usually from the sun, striking the PV panel. In order to maximize the transfer of available energy from the PV panel, a load connected to the panel must have a resistance value that provides maximum power transfer. Furthermore, the value of that resistance should be controlled in order to maintain an impedance match with the output impedance of the PV panel as the output characteristics of the PV panel change.
In general, a PV system for providing electrical energy comprises PV panels, DC power converters, and controllers. For other PV systems, the energy sources may be combinations of PV panels and PV cells. A DC power converter is coupled to each PV panel and is used to convert the panel output voltage and current to a load voltage and load current. The load voltage is applied to a load, such as a battery, heater or other device. The output of the converter is controlled by a controller so that the load voltage has a desired value and the load current has values dependent on the power available from the respective PV panel.
For most PV systems, a dedicated converter serves as a variable load to each PV panel and has an output suitable for supplying energy to a load. The output of the PV panel is coupled to the input of the converter. If the controller of the converter operates using a maximum power point tracker (MPPT) algorithm, then most of the energy available from a panel is available (except for losses in converter components) at the output of the converter. It is well known that when an MPPT algorithm of a controller controls the operation of the converter, then maximum power is available to a load connected to the converter. For a typical solar power system, a dedicated controller and converter are assigned to each solar panel. The controller monitors the output voltage and output current of its respective solar panel and uses an MPPT algorithm to adjust the duty cycle of the converter in order to obtain maximum power transfer. Because the input resistance of the converter is the resistance seen by the solar panel, that resistance should change in response to the panel changes in order to maximize power transfer. By varying the duty cycle of the converter, the input impedance of the converter changes in such a way that maximum power is transferred to a load connected to the output of the converter.
A power converter is a device that converters one voltage value to another voltage value. Power converters, such as switching DC-to-DC converters, are capable converting a variety of input voltages to a desired output voltage and output current. These switching DC-to-DC converters, hereafter called converters, are often used with alternative energy sources such as PV panels and wind turbines. Such converters have a controller that responds quickly to the changing output characteristics of the alternative energy source in order to extract maximum energy. Although embodiments of a controller described in this disclosure are used to extract maximum energy from multiple PV panels, the controller may be used by other energy sources. Other types of converters, sometimes called inverters, may convert outputs from PV panels into AC voltages that are coupled to a power distribution grid.
It is understood by those working in solar energy technology that shadows from clouds can cause sudden changes in the available energy from a PV panel so that any device that extracts maximum power must adapt to these changing conditions. Converters capable of adapting to the output changes of a PV panel usually have a controller that uses the MPPT algorithm. Controllers that use the MPPT algorithm cause a converter to serve as variable resistance to the PV panel in such a way that power transfer is at a maximum. Controllers that use the MPPT algorithm are complex and must be connected to sensors that continuously monitor voltages and currents. As with most technologies complexity is directly related to cost, so that an improved control system could reduce the cost of producing solar energy.
A PV panel 10 is depicted in FIG. 1. The PV panel 10 comprises multiple PV cells 20 that are connected together so as to provide an output voltage and current at output port 12. The output voltage of the PV panel across the two conductors of the port is VP, and the output current, IP, flows from the port when a load (not shown) is connected across the port. Since power, P=V*I, then the maximum power point (MPP) occurs at the knee of the PV output curve depicted in FIG. 2. At the MPP, the resistance of a load must equal Vmax/Imax. When no load is connected to the PV panel the panel voltage is VOC and when the output terminals are short circuited the current flowing is ISC. Note that Vmax is less than VOC and Imax is less than ISC. For conventional solar energy systems, a dedicated converter and MPPT controller is assigned to each PV panel of the system so that maximum energy is extracted from the system despite uneven characteristic of the outputs of the PV panels.